1. Technical Field
The inventive arrangements relate generally to methods and apparatus for providing improvements to planar antenna elements in an array, and more particularly to reducing the undesirable effects caused by mutual coupling among adjacent array elements.
2. Description of the Related Art
Phased array antenna systems are well known in the antenna art. Such antennas are generally comprised of a plurality of radiating elements that are individually controllable with regard to relative phase and amplitude. The antenna pattern of the array is selectively determined by the geometry of the individual elements and the selected phase/amplitude relationships among the elements. Typical radiating elements for such antenna systems may be comprised of dipoles, slots or any other suitable arrangement.
In recent years, a variety of new planar type antenna elements have been developed which are suitable for use in array applications. One example of a planar antenna element is disclosed in U.S. Pat. No. 5,926,137 to Nealy, the disclosure of which is hereby incorporated by reference. The planar type antenna-radiating element disclosed therein is commonly known in the art as the Foursquare antenna. The design is a dual polarized, moderately wideband element that consists of a printed metalization on a low loss substrate suspended over a ground plane reflector. Various polarizations can be achieved with the Foursquare element. For example, dual linear, circular and elliptical polarizations of any orientation or sense are possible. The Foursquare element can be arranged into an array to produce a highly directive beam. The array beam can then be scanned by adjusting the relative phase of the elements according to conventional practice.
Broadband array antennas offer many benefits as compared to narrow band arrays in a wide variety of applications ranging from wireless broadband communications systems to radar systems for the military. However, broadband arrays are known to be difficult to design due to certain conflicting design criteria. Most notable among these are the challenges associated with selection of suitable broadband antenna radiating elements. In addition, close spacing of certain planar antenna elements in an array has proven to be a problem due to the mutual coupling in the array among the individual elements. Such coupling can be used advantageously to achieve wider bandwidths than would otherwise be possible for individual elements. However, the mutual coupling which allows increased performance with regard to bandwidth can also have certain negative effects. For example, such mutual coupling may distort theoretical antenna patterns where the effect of coupling is not included and change the input impedance of individual elements at a selected operating frequency.
Some research efforts have attempted to deal with the effects of mutual coupling in the array context by addressing these issues in the initial design of the individual array elements. However, this creates an added level of design complexity that is undesirable in many systems. What is needed is an improved arrangement for reducing the mutual coupling effect without substantially increasing the size or weight of the radiating elements. For example, it has been found that mutual coupling has been reduced in the case of some kinds of array elements by positioning the element in a cavity. Problems with this approach include increased cost and weight, as well as a greater complexity in the mechanical design of the array.
The invention concerns a method and apparatus for reducing mutual coupling among adjacent planar antenna radiating elements in an array. The elements can be positioned adjacent to one another in a standard geometric array configuration. A circumferential conductive metal line is provided in the plane of each element at an outer perimeter thereof. The conductive metal line is electrically connected to a ground potential. The ground plane potential is preferably provided by a ground plane reflector over which the antenna elements are suspended. The conductive metal line can be connected to the ground plane reflector by one or more ground posts extending between the conductive metal line and the ground plane reflector.
The individual antenna elements comprising the array can be formed of a radiating element portion provided on a dielectric layer. For example, the radiating element can be etched from a copper cladding formed on the dielectric layer. The conductive line can also be etched from the copper cladding so that the radiating element and the line are in a common plane. According to one embodiment, the radiating element portion can be a Foursquare antenna radiating element.
The invention can also include an individual antenna element for providing reduced coupling when positioned among a plurality of adjacent antenna elements in an array. In that case, the individual antenna element comprises a dielectric layer, a radiating element formed on the dielectric layer, and a circumferential conductive metal line in the plane of the radiating element. The radiating element can be formed as a Foursquare type element, but the invention is not so limited. The circumferential metal line can be spaced from the radiating element to form an outer perimeter thereof. The conductive metal line is connected to a ground potential such as a ground plane reflector over which the element is suspended. The circumferential conductive metal line can be electrically connected to the ground plane by at least one ground post extending between the conductive metal line and the ground plane reflector. According to one aspect of the invention, the antenna radiating element and the conductive metal line are each formed from a copper cladding on the dielectric layer so that they form a common plane.
The invention can also include a scannable array of planar radiating elements having reduced mutual coupling. The radiating elements can be formed in a Foursquare configuration, but the invention is not so limited. According to one embodiment, a plurality of the antenna elements as described herein can be arranged adjacent to one another in an array configuration with a plurality of feed points connected to the radiating elements. An RF controller can be provided for controlling at least one of a phase and amplitude of RF applied to the radiating elements at the feed points. A circumferential conductive metal line can be provided in the plane of each the element at an outer perimeter thereof and is advantageously connected to a ground potential such as a ground plane reflector over which the element is suspended. The connection to the ground plane can be provided by one or more ground posts extending between the conductive metal line and the ground plane reflector. At least one of the radiating elements can be a foursquare antenna radiating element, but the invention can also be implemented with a variety of other well known antenna radiating elements.